Knee prosthesis with a rotational plate

ABSTRACT

The knee prosthesis with rotary plate comprises a metal base ( 2 ) and a tibia plate ( 3 ) which are equipped with guide means ( 22, 22′, 22″, 26, 29, 5, 6, 5′, 9, 50, 51, 52, 13, 15, 18; 34, 34′, 34″, 35, 37, 7, 8, 10, 12, 53, 54, 55, 10′, 17 ) defining a center of rotation (C, C′) which may be offset from that of the tibia bone axis (YY′), so as to allow the tibia plate ( 3 ) to slide in rotation over the said base, the said guide means being positioned a certain distance away from the center of rotation (C, C′).

[0001] The present invention relates to a knee prosthesis and morespecifically to the device for assembling its tibia plate, which is madeof plastic, on its metal base anchored beforehand into the tibia bonetissue.

[0002] Knee prostheses which comprise a plastic tibia part which is freeto rotate about the tibia bone axis with respect to the metal basesecured to the tibia are known.

[0003] This freedom of rotation is generally achieved via a male shaftintegral with the plastic tibia part which engages with a bore made inthe metal base.

[0004] Furthermore, this freedom of rotation may be achieved by a maleshaft secured to the metal base which engages with a bore made in theplastic tibia part.

[0005] These connecting devices have certain drawbacks, namely thedifficulty of inserting the plastic tibia plate once the metal base isin place on the tibia. These drawbacks are exacerbated when theposterior cruciate ligament is being kept, as this restricts access tothe upper part of the tibia.

[0006] Furthermore, it is observed that the center of rotation isnecessarily positioned at a point of the device or [sic] the bore can bemade, that is to say in a part or [sic] there is enough material to makethe said bore. This arrangement of the center of rotation is notstrictly anatomical.

[0007] It can be seen that the volume generated by the tibia bone axismay prevent a posterior cutout from being made in the plastic tibiaplate or in the metal base to allow the posterior cruciate ligament tobe kept.

[0008] Knee prostheses which comprise a plastic tibia plate which slidesfreely over the flat surface of the metal base, and the movement ofwhich is limited by one or more cylindrical studs integral with the basecommunicating with spaces made in the said plastic plate are also known.

[0009] This type of prosthesis has the drawback of not physicallyembodying an axis of rotation.

[0010] It is these drawbacks which the present invention sets out moreparticularly to overcome.

[0011] The knee prosthesis according to the present invention isintended to provide a plastic tibia plate which has a degree of freedomin rotation with respect to the metal base.

[0012] The knee prosthesis in accordance with the present inventioncomprises a metal base and a tibia plate which are equipped with guidemeans defining a center of rotation which may be offset from that of thetibia bone axis, so as to allow the tibia plate to slide in rotationover the said base, the said guide means being positioned a certaindistance away from the center of rotation.

[0013] The knee prosthesis has guide means which consist of at least oneupstand in the shape of an arc of a circle secured to the metal base andof a housing with the same radius of curvature made in the plastic tibiaplate to allow the latter to slide in rotation about the center ofrotation of the said upstand.

[0014] The knee prosthesis according to the present invention comprisesguide means which consist of an upstand in the shape of an arc of acircle, which upstand is positioned in the anterior part of the metalbase and oriented in a substantially medio-lateral direction.

[0015] The knee prosthesis according to the present invention comprisesadditional guide means which are positioned on or near to the center ofrotation of the tibia plate on the metal base.

[0016] The knee prosthesis according to the present invention comprisesadditional guide means which are secured to a device making it possibleto prevent the tibia plate from lifting from the metal base.

[0017] The knee prosthesis in accordance with the present inventioncomprises guide means which consist of at least two pegs set out in anarc of a circle and defining a center of rotation, and of a housing ofthe same radius of curvature formed in the tibia plate.

[0018] The knee prosthesis according to the present invention comprisesa metal base which comprises an upstand in the shape of an arc of acircle having a central part secured to lateral edges which are not astall as the said central part, while the tibia plate comprises, on itslower face, a housing in the shape of an arc of a circle.

[0019] The knee prosthesis according to the present invention comprisesan upstand which has a center of rotation which is borne by the tibiabone vertical axis, while the said upstand is a certain distance awayfrom its center of rotation.

[0020] The knee prosthesis according to the present invention comprisesan upstand which has a center of rotation which is offset from the tibiabone vertical axis, while the said upstand is a certain distance awayfrom its center of rotation.

[0021] The knee prosthesis according to the present invention comprisesa metal base which has two upstands in the shape of an arc of a circle,of constant height and having one and the same center of rotation, whilethe tibia plate comprises two housings in the shape of an arc of acircle.

[0022] The knee prosthesis according to the present invention comprisesupstands which are set out opposite one another, and have one and thesame center of rotation.

[0023] The knee prosthesis according to the present invention comprisesa metal base which has, opposite the upstand, a retaining peg borne by acenter of rotation so as to engage with a cutout formed in the tibiaplate to prevent the latter from lifting off the base as the said plateslides in rotation about its center of rotation.

[0024] The knee prosthesis according to the present invention comprisesa retaining peg which consists of a cylindrical pin integral with a headwhich has a larger diameter than the said pin so that the said headengages with inclined faces made in the cutout.

[0025] The knee prosthesis according to the present invention comprisesa metal base which has, opposite the upstand, a centering peg borne bythe center of rotation so as to engage with a blind hole formed in thetibia plate to guide the latter with respect to the base as the saidplate slides in rotation about its center of rotation.

[0026] The knee prosthesis according to the present invention comprisesa metal base and a tibia plate which respectively comprise a cutoutthrough which the posterior cruciate ligament can pass.

[0027] The knee prosthesis according to the present invention comprisesa metal base which has two upstands in the shape of an arc of a circlecurved in the same direction and centered about the same center ofrotation, while the tibia plate comprises housings intended to receivethe said upstands respectively, so as to allow the said plate to slidein rotation about the center of rotation.

[0028] The knee prosthesis according to the present invention comprisesan upstand which is integral with a flange which engages in a slot inthe housing to prevent the tibia plate from lifting off the metal baseas the said plate slides in rotation about the center of rotation.

[0029] The knee prosthesis according to the present invention comprisesa metal base which comprises two upstands in the shape of an arc of acircle in opposite directions and centered about the same center ofrotation, while the tibia plate comprises an element and a housing whichare intended to receive the said upstands respectively to allow the saidplate to slide in rotation about the center of rotation.

[0030] The knee prosthesis according to the present invention comprisesan upstand which is set out on the external periphery of the horizontaldisk of the metal base so as to engage with a peripheral recess in thetibia plate.

[0031] The knee prosthesis according to the present invention comprisesan upstand which is offset from the center of rotation and comprises aflange which snap-fastens into the housing in the tibia plate to, on theone hand, guide the plate as it slides in rotation about its center and,on the other hand, retain the said plate so that it does not lift offthe metal base.

[0032] The knee prosthesis according to the present invention comprisesa metal base which comprises a peripheral upstand in the shape of an arcof a circle integral with a flange directed toward the tibia bonevertical axis and a housing set out in the region of the center ofrotation, while the tibia plate has, on its external periphery, a recessin which there is formed a horizontal slot intended to receive theflange of the said upstand and, on its lower face, a stub which engageswith the housing.

[0033] The knee prosthesis according to the present invention comprisesa metal base which comprises three peripheral upstands extendingvertically above the horizontal disk, while the tibia plate has, on itsexternal periphery, three recesses intended to receive the said upstandsrespectively to allow the said plate to be guided as it slides inrotation about the center of rotation.

[0034] The knee prosthesis according to the present invention comprisespegs which are set out in an arc of a circle about a center of rotation,while the tibia plate has a housing intended to receive the said pegs.

[0035] The knee prosthesis according to the present invention comprisespegs which have a center of rotation which is borne by the tibia bonevertical axis, while the said peg is a certain distance away from itscenter of rotation.

[0036] The knee prosthesis according to the present invention comprisespegs which have a center of rotation which is offset from the tibia bonevertical axis, while the said peg is a certain distance away from itscenter of rotation.

[0037] The knee prosthesis according to the present invention comprisesa metal base which comprises at least one upstand or peg which engageswith a housing of the tibia plate so that the said plate can slide inrotation over the metal base only within the limit set by the differencein size between the said upstand or peg and the said correspondinghousing.

[0038] The knee prosthesis according to the present invention has arotational travel between the tibia plate and the metal base which isreduced to zero when the dimensions of the housing are made so as toengage without clearance with the upstand.

[0039] The knee prosthesis according to the present invention has ashort height of the guide means and of their anterior positioning on themetal base which allows the tibia plate to be mounted on the said basevia a strictly anterior approach, the said plate requiring upwardclearance only by the height of the said guide means.

[0040] The description which will follow with reference to the appendeddrawings, given by way of nonlimiting examples, will allow a goodunderstanding of the invention, its features and the advantages it islikely to afford:

[0041]FIG. 1 is an exploded perspective view illustrating the kneeprosthesis according to the present invention.

[0042]FIGS. 2 and 3 are side views showing the knee prosthesis beforethe plastic tibia plate is fitted on the metal base.

[0043]FIGS. 4 and 5 are views depicting a first alternative form of theknee prosthesis according to the invention.

[0044]FIGS. 6, 7 and 8 are views illustrating a second alternative formof the knee prosthesis in which the metal base has two opposed guideupstands.

[0045]FIGS. 9, 10 and 11 are views showing a third alternative form ofthe knee prosthesis in which the metal base has, on its axis ofrotation, a peg for retaining the plastic tibia plate.

[0046]FIGS. 12, 13 and 14 are views showing a fourth alternative form ofthe knee prosthesis in which the metal base has, on its axis ofrotation, an additional peg for centering the plastic tibia plate.

[0047]FIGS. 15, 16 and 17 are views depicting a fifth alternative formof the knee prosthesis which has a cutout through which the posteriorcruciate ligament can pass.

[0048] FIGS. 18 to 24 are views illustrating alternative forms of theknee prosthesis in which the metal base has two opposed guide upstandsbut one of which is borne by the center of rotation of the second.

[0049] FIGS. 25 to 27 are views depicting other alternative forms of theknee prosthesis according to the present invention.

[0050]FIGS. 28 and 29 are views illustrating guide means set out at theperiphery of the metal base and of the tibia plate of the kneeprosthesis.

[0051] FIGS. 30 to 32 are views showing guide means consisting of atleast two vertical pegs integral with the metal base and which engagewith a housing formed in the tibia plate.

[0052] FIGS. 1 to 3 show a knee prosthesis 1 comprising a metal base 2and a tibia plate 3, whereas the femoral element is not depicted.

[0053] The metal base 2 consists of a horizontal disk 20 secured on oneof its faces to an anchoring rod 21 allowing the said base to be fixedinto the tibia of a patient.

[0054] The horizontal disk 20 comprises, on the opposite side to the rod21, guide means which consist of an upstand 22 with an exterior profilein the shape of an arc of a circle. In this case, note that the centerof rotation C of the upstand 22 is borne by the tibia bone vertical axisYY′.

[0055] The upstand 22 extending vertically above the horizontal disk 20has a central part 23 integral on each side with two vertical edges 24and 25 which are not as tall as the said central part.

[0056] In addition, the upstand 22 is positioned on the horizontal disk20 of the metal base 2 a certain distance away from the center ofrotation C.

[0057] The tibia plate 3, which is made of plastic, has a flat lowerface 30 arranged in a horizontal plane parallel to the plane containingthe disk 20 of the metal base.

[0058] On the opposite side to the face 30 the tibia plate 3 has anupper face 31 with two tracks 32 and 33 of concave profile which areintended to receive the condyles of the femoral element, not depicted.

[0059] The lower face 30 is pierced with a housing 34 with the sameradius of curvature as the upstand 22 secured to the metal plate 2.

[0060] The housing 34 has cutaways 35 and 36 of a larger size which areintended to receive respectively the central part 23 and the lateraledges 24 and 25 of the upstand 22.

[0061] It can be seen that the tibia plate 3, and more particularly itslower face 30, is resting on the horizontal disk 20 of the metal base 2,that the upstand 22 enters the housing 34.

[0062] The latter has dimensions that exceed those of the upstand 22 sothat the tibia plate 3 can slide freely in rotation about the center ofrotation C of the said upstand 22 in the direction of the arrow Fillustrated in FIG. 1.

[0063] The upstand 22, which is in the shape of an arc of a circle orcurved, is positioned in the anterior part of the metal base 2 andoriented in a substantially medio-lateral direction.

[0064]FIGS. 4 and 5 show a first alternative form of the knee prosthesis1 according to the present invention.

[0065] The knee prosthesis 1 is identical to the one described earlier,that is to say that it comprises a metal base 2 and a plastic tibiaplate 3.

[0066] The metal base 2 has, and this is what differentiates it from theone described above, the position of the guide means on the horizontaldisk 20. In effect, the guide means consist of an upstand 22 in theshape of an arc of a circle and the profile of which is similar to theone described earlier, but the center of rotation C′ of which is offsetfrom that C borne by the tibia bone vertical axis YY′.

[0067] Note that the center of rotation C′ can be positioned anywhere,either on the horizontal disk 20 or off it, while at the same timekeeping the guide means on the said horizontal disk and morespecifically at a particular point.

[0068] Just as before, the upstand 22 provided with its central part 23and its lateral edges 24 and 25 enters the housing 34 formed in thelower face 30 of the plastic tibia plate 3 to allow the latter to slidein rotation over the metal base 2 and about the center of rotation C′ inthe direction of the arrow F1 in FIG. 4.

[0069] The upstand 22, which is in the shape of an arc of a circle orcurved, is positioned in the anterior part of the metal base 2 andoriented in a substantially medio-lateral direction.

[0070] FIGS. 6 to 8 depict a second alternative form of the kneeprosthesis 1 according to the invention.

[0071] The latter comprises a metal base 2 and a plastic plate 3 similarto those described in FIG. 1, but in which the guide means differ toallow for a better movement of the plate over the base.

[0072] Thus, the metal base 2 equipped with its horizontal disk 20integral with its rod 21 comprises guide means which consist of twoopposed upstands 22′ and 22″ in the shape of an arc of a circle. Theupstands 22′ and 22″ have the same center of curvature which iscoincident with the center of rotation C located on the tibia bonevertical axis YY′.

[0073] Provision may be made for the center of rotation of the upstands22′ and 22″ to differ from the one C, that is to say offset as shown inthe first alternative form in FIGS. 4 and 5.

[0074] Note too that the upstands 22′ and 22″ have a constant heightover the entire circular-arc-shaped profile.

[0075] The tibia plate 3 has a flat lower face 30 with two opposedhousings 34′ and 34″ intended to receive respectively the upstands 22′and 22″ of the metal base 2. The housings 34′ and 34″ have a radius ofcurvature identical to that of the upstands 22′ and 22″.

[0076] It can be seen when the tibia plate 3, and more specifically itslower face 30, is resting on the horizontal disk 20 of the metal base 2,that the upstands 22′ and 22″ enter the housings 34′ and 34″.

[0077] The latter have dimensions greater than those of the upstands 22′and 22″ so that the tibia plate 3 can slide freely in rotation about thecenter of rotation C in the direction of the arrow F3 illustrated inFIG. 6.

[0078] FIGS. 9 to 11 illustrate a third alternative form of the kneeprosthesis 1 according to the present invention.

[0079] The knee prosthesis 1 is identical to the one described in FIGS.1 to 3, namely it comprises a metal base 2 and a plastic tibia plate 3.

[0080] The metal base 2 comprises, on its horizontal disk 20, and on theopposite side to its anchoring rod 21, guide means which consist of theupstand 22 formed from a central part 23 and of two lateral edges 24 and25.

[0081] Furthermore, the horizontal disk 20 comprises, at the center ofrotation C of the upstand 22 which is borne by the tibia bone verticalaxis YY′, a retaining peg 26 extending vertically above the said disk20.

[0082] The retaining peg 26 constitutes a guide means that is inaddition to the guide means formed by the upstand 22. Thus, the peg 26is positioned on the center of rotation C of the tibia plate 3 on themetal base 2.

[0083] The retaining peg 26 consists of a cylindrical pin 27 integralwith a head 28 whose outside diameter exceeds that of the said pin.

[0084] The additional guide means or peg 26 is an integral part of thepin 27, 28 to prevent the tibia plate 3 from lifting off the metal base2 when the prosthesis is in motion.

[0085] The tibia plate 3 has, on its lower face 30, the housing 34 thatreceives the upstand 22 and a cutout 35 with inclined faces 36. When theplate 3 is mounted on the base 2, this cutout engages with the retainingpeg 26 so that the head 28 lies above the inclined faces 36.

[0086] It can be seen that the retaining peg 26, when it engages withthe cutout 35, allows the tibia plate 3 to be prevented from liftingunder a tensile force when the said plate is sliding in rotation F4 onthe metal base 2.

[0087] FIGS. 12 to 14 show a fourth alternative form of the kneeprosthesis 1 according to the invention.

[0088] Thus, the metal base 2 comprises, opposite the upstand 22, and inthe region of the center of rotation C which is borne by the tibia bonevertical axis YY′, a centering peg 29 extending vertically above thehorizontal disk 20.

[0089] The centering peg 29 extends vertically above the horizontal disk20 by a short height, constituting a guide means that is in addition tothe guide means formed by the upstand 22.

[0090] The centering peg 29 consists of a short cylindrical pin.

[0091] The plastic tibia plate 3 comprises, on its lower face 30 andopposite the housing 34, a blind hole 37 intended to receive thecentering peg 29 when the said plate is fitted onto the metal base 2.

[0092] The centering peg 29 provides a physical embodiment of the centerof rotation C of the upstand 22 as the tibia plate 3 slides in rotationover the metal base 2.

[0093] The knee prosthesis 1 illustrated in FIGS. 15 to 17 differs fromthe one shown in FIGS. 1 to 3 only in the fact that the metal base 2 andthe plastic tibia plate 3 respectively comprise a cutout 4 and 38 forthe passage of the posterior cruciate ligament.

[0094] Quite obviously, the prosthesis 1 shown in FIGS. 15 to 17 worksin the same way as the one described in FIGS. 1 to 3.

[0095]FIG. 18 shows the knee prosthesis 1 equipped with its metal base 2and with its tibia plate 3. The metal base 2 comprises, on itshorizontal disk 20, and more specifically on the opposite side to theanchoring rod 21, an upstand 5 in the shape of an arc of a circle and ofvariable height. This upstand has a profile which differs from thosethat make up the upstands described earlier. At the center of rotation Cof the upstand 5 there is a second upstand 6 in the shape of an arc of acircle.

[0096] The upstands 5 and 6 engage in housings, not depicted, but formedon the face 30 of the tibia plate 3 to allow the said plate to be guidedas it slides in rotation about the center of rotation C, as depicted bythe arrow F5.

[0097] The upstand 5, in the shape of an arc of a circle or curved, ispositioned in the anterior part of the metal base 2 and oriented in asubstantially medic-lateral direction.

[0098] The upstand 6 constitutes a guide means that is in addition tothe guide means formed by the upstand 5. Thus, the upstand 6 ispositioned on the center of rotation C of the tibia plate 3 on the metalbase 2.

[0099] Provision may be made for the center of rotation of the upstands5 and 6 to be offset from the tibia bone vertical axis YY′ without thisin any way altering the subject of the invention.

[0100] FIGS. 19 to 21 depict an alternative form of the knee prosthesis1 shown in FIG. 18, namely wherein the upstand 6 is integral with aflange 60 forming a kind of small plate set out in a horizontal planeparallel to the plane containing the disk 20 of the metal base 2.

[0101] The tibia plate 3 has, on its face 30, housings 7 and 8 intendedto receive respectively the upstands 5 and 6 to allow the tibia plate 3to slide in rotation over the metal base 2 about the center of rotationC and in the direction of the arrow F5.

[0102] The housing 7 has a profile essentially identical to that of theupstand 5, and at the very least, in the shape of an arc of a circle forguiding the tibia plate 3 in its travel.

[0103] The housing 8 has a profile essentially identical to the flange60 of the additional guide means or upstand 6 for guiding the tibiaplate 3 in its travel.

[0104] The housing 8 is pierced with an internal slot 80 intended toreceive the flange 60 of the upstand 6 to achieve a kind ofsnap-fastening of the tibia plate 3 to the base 2, so that the saidplate cannot lift under a tensile force.

[0105] Note that the upstands 5 and 6 in the shape of an arc of a circleare curved in the same direction and about the same center of rotation Cor C′ when the latter is offset from the tibia bone vertical axis YY′.

[0106] As in FIG. 15, the metal base 2 and the tibia plate 3 mayrespectively comprise cutouts 4 and 38 for the passage of the posteriorcruciate ligament.

[0107] In FIGS. 22 to 24 the knee prosthesis 1 comprises, on its metalbase 2, a first upstand 5′ set out at the external periphery of thehorizontal disk 20. Opposite the upstand 5′ the horizontal disk 20 issecured to another upstand 9 in the shape of an arc of a circle, but theradius of curvature of which is inverted compared with that of theupstand 5′.

[0108] The upstand 5, which is in the shape of an arc of a circle orcurved, is positioned in the anterior part of the metal base 2 andoriented in a substantially medio-lateral direction.

[0109] The upstand 9 constitutes a guide means that is in addition tothe guide means formed by the upstand 5′. Thus, the upstand 9 ispositioned on the center of rotation C of the tibia plate 3 on the metalbase 2.

[0110] The upstand 9 has the same center of rotation C as the upstand5′, but the said center may be offset, depending on the configuration ofthe knee prosthesis, from the tibia bone vertical axis YY′.

[0111] The upstand 9 is integral with a flange 90, the function of whichwill be seen more clearly later.

[0112] The tibia plate 3 has, on its lower periphery, that is to say theone that lies between the faces 30 and 31, a recess 10 receiving theupstand 5′ when the tibia plate 3 is fitted onto the metal base 2.

[0113] On the opposite side to the recess 10, the lower face 30 ispierced with a housing 12 into which the upstand 9 can be snap-fastenedto, on the one hand, guide the plate 3 as it slides in rotation aboutits center C, and, on the other hand, retain the said plate to preventit from lifting off the metal base 2.

[0114] In FIGS. 25 to 27 the knee prosthesis 1 comprises the metal base2, the horizontal disk 20 of which has, on its external periphery, anupstand 13 in the shape of an arc of a circle integral with a flange 14directed toward the tibia bone vertical axis YY′.

[0115] The horizontal disk 20 is pierced at the center of rotation C ofthe upstand 13 with a dish-shaped housing which constitutes a guidemeans that is in addition to the guide means formed by the said upstand13. This upstand, which is in the shape of an arc of a circle or curved,is positioned in the anterior part of the metal base 2 and oriented in asubstantially medio-lateral direction.

[0116] The upstand 13 and the housing 15 may comprise a center ofrotation C which is offset from the tibia bone vertical axis YY′.

[0117] On its external periphery and between the faces 30 and 31, thetibia plate 3 has a recess 10′ in which there is formed a horizontalslot 16 intended to receive the flange 14 of the upstand 13 as the plate3 slides in rotation over the metal base 2.

[0118] The lower face 30 is integral with a stub 17 with a conicaltapering profile capable of engaging with the housing 15 formed in thehorizontal disk 20 of the metal base 2.

[0119] The upstand 13 equipped with its flange 14, the housing 15, therecess 10′ and its slot 16, and the stub 17 constitute the means ofguiding the tibia plate 3 over the metal base 2 as the said plate slidesin rotation in the direction of the arrow F5.

[0120] In FIGS. 28 and 29, the metal base 2 of the knee prosthesis 1comprises, on its horizontal disk 20, three peripheral upstands 50, 51and 52 extending vertically above the said horizontal disk 20.

[0121] The tibia plate 3 comprises, on its external periphery, threeperipheral recesses 53, 54 and 55 which are intended to receive theupstands 50, 51 and 52 respectively to allow the said plate to be guidedas it slides in rotation about the center of rotation of the saidupstands, which is identical for all three.

[0122] The upstand 51, which is the shape of an arc of a circle orcurved, is positioned in the anterior part of the metal base 2 andoriented in a substantially medio-lateral direction.

[0123] The upstands 50, 52 in the shape of an arc of a circle constituteguide means which are in addition to the guide means formed by theupstand 51. The upstands 50, 52 are positioned near the center ofrotation C of the tibia plate 3 on the metal base 2.

[0124] In FIGS. 30 to 32, the metal base 2 of the knee prosthesis 1comprises, on its horizontal disk 20, vertical pegs 18 set out in an arcof a circle about a center of rotation C which may be either borne by oroffset from the tibia bone vertical axis YY′.

[0125] The pegs 18 set out in an arc of a circle are positioned in theanterior part of the metal base 2 and oriented in a substantiallymedio-lateral direction.

[0126] In this example, the tibia plate 3 is identical to the onedescribed in FIGS. 1 to 3, that is to say that its lower face 30 has ahousing 34 intended to receive the pegs 18 for guiding the said plate asit slides in rotation with respect to the metal base 2.

[0127] It can be seen that in each example described hereinabove it ispossible, as the case may be, for the center of rotation of the upstandsor of the pegs either to be offset or not to be offset in order todetermine a particular rotational sliding of the tibia plate 3 withrespect to the metal base 2.

[0128] Note too that in each example described hereinabove, cutouts 4and 38 may or may not be provided for the passage of the posteriorcruciate ligament.

[0129] Furthermore, it can be seen that the guide means in each exampledescribed hereinabove are positioned a certain distance away from thecenter of rotation C, C′.

[0130] It will be observed that the short height of the upstand or ofthe pegs, and its anterior position on the metal base 2 allows theplastic plate 3 to be fitted onto the said base easily using a strictlyanterior approach, the plate requiring upward clearance only by theheight of the said upstand or of the said pegs as shown in FIG. 21.

[0131] It will furthermore be observed that the plastic plate 3 cannotslide in rotation over the metal base 2 except within the limits set bythe difference in dimensions between the housing and the correspondingupstand or peg, and this prevents any undesired excess movement.

[0132] This being so, it will be readily understood that a plastic plate3 can be obtained in which the dimensions of the housing are identicalto those of the upstand or of the corresponding pegs, so as to preventany travel of the said plate over the metal base 2. This allows thesurgeon, according to the particular surgical case, to return to a kneeprosthesis system with plastic plate which is fixed to the metal base 2,without having to change the latter.

1. Knee prosthesis comprising a metal base secured to an anchoring rodfor fixing it into the tibia of a patient, and a plastic tibia platewhich can slide freely over the said base, characterized in that themetal base (2) and the tibia plate (3) are equipped with guide means(22, 22′, 22″, 26, 29, 5, 6, 5′, 9, 50, 51, 52, 13, 15, 18; 34, 34′,34″, 35, 37, 7, 8, 10, 12, 53, 54, 55, 10′, 17) defining a center ofrotation (C, C′) which may be offset from that of the tibia bone axis(YY′), so as to allow the tibia plate (3) to slide in rotation over thesaid base, the said guide means being positioned a certain distance awayfrom the center of rotation (C, C′):
 2. Knee prosthesis according toclaim 1 , characterized in that the guide means consist of at least oneupstand (22, 22′, 22″, 5, 6, 5′, 9, 50, 51, 52, 13) in the shape of anarc of a circle secured to the metal base (2) and of a housing (34, 34′,34″, 7, 8, 10, 12, 53, 54, 55, 10′) with the same radius of curvaturemade in the tibia plate (3) to allow the latter to slide in rotationabout the center of rotation (C, C′) of the said upstand.
 3. Kneeprosthesis according to claim 2 , characterized in that the guide meansconsist of an upstand (22, 5, 5′, 51, 13) in the shape of an arc of acircle, which upstand is positioned in the anterior part of the metalbase (2) and oriented in a substantially medio-lateral direction. 4.Knee prosthesis according to claim 1 , characterized in that theadditional guide means (26, 29, 6, 9, 15, 50, 52) are positioned on ornear to the center of rotation (C, C′) of the tibia plate (3) on themetal base (2).
 5. Knee prosthesis according to claim 4 , characterizedin that the additional guide means (26, 6, 9) are secured to a device(28, 60, 90) making it possible to prevent the tibia plate (3) fromlifting from the metal base (2).
 6. Knee prosthesis according to claim 1, characterized in that the guide means consist of at least two pegs(18) set out in an arc of a circle and defining a center of rotation (C,C′), and of a housing (34) of the same radius of curvature formed in thetibia plate (3), the said pegs being positioned in the anterior part ofthe metal base (2) and oriented in a substantially medio-lateraldirection.
 7. Knee prosthesis according to claim 2 , characterized inthat the metal base (2) comprises an upstand (22) in the shape of an arcof a circle having a central part (23) secured to lateral edges (24, 25)which are not as tall as the said central part, while the tibia plate(3) comprises, on its lower face (30), a housing (34) in the shape of anarc of a circle.
 8. Knee prosthesis according to claim 2 , characterizedin that the upstand (22, 22′, 22″, 5, 6, 5′, 9, 50, 51, 52, 13) has acenter of rotation (C) which is borne by the tibia bone vertical axis(YY′), while the said upstand is a certain distance away from its centerof rotation.
 9. Knee prosthesis according to claim 2 , characterized inthat the upstand (22, 22′, 22″, 5, 6, 5′, 9, 50, 51, 52, 13) has acenter of rotation (C′) which is offset from the tibia bone verticalaxis (YY′), while the said upstand is a certain distance away from itscenter of rotation.
 10. Knee prosthesis according to claim 2 ,characterized in that the metal base (2) has two upstands (22′, 22″) inthe shape of an arc of a circle, of constant height and having one andthe same center of rotation (C, C′), while the tibia plate (3) comprisestwo housings (34′, 34″) in the shape of an arc of a circle.
 11. Kneeprosthesis according to claim 10 , characterized in that the upstands(22′, 22″) are set out opposite one another, and have one and the samecenter of rotation (C, C′).
 12. Knee prosthesis according to claim 1 ,characterized in that the metal base (2) has, opposite the upstand (22),a retaining peg (26) borne by a center of rotation so as to engage witha cutout (35) formed in the tibia plate (3) to prevent the latter fromlifting off the base (2) as the said plate slides in rotation about itscenter of rotation.
 13. Knee prosthesis according to claim 11 ,characterized in that the retaining peg (26) consists of a cylindricalpin (27) integral with a head (28) which has a larger diameter than thesaid pin so that the said head engages with inclined faces made in thecutout (35).
 14. Knee prosthesis according to claim 1 , characterized inthat the metal base (2) has, opposite the upstand (22), a centering peg(29) borne by the center of rotation (C, C′) so as to engage with ablind hole (37) formed in the tibia plate (3) to guide the latter withrespect to the base (2) as the said plate slides in rotation about itscenter of rotation.
 15. Knee prosthesis according to claim 1 ,characterized in that the metal base (2) and the tibia plate (3)respectively comprise a cutout (4 and 38) through which the posteriorcruciate ligament can pass.
 16. Knee prosthesis according to claim 1 ,characterized in that the metal base (2) has two upstands (5 and 6) inthe shape of an arc of a circle curved in the same direction andcentered about the same center of rotation (C, C′), while the tibiaplate (3) comprises housings (7 and 8) intended to receive the saidupstands (5 and 6) respectively, so as to allow the said plate to slidein rotation about the center of rotation (C, C′).
 17. Knee prosthesisaccording to claim 16 , characterized in that the upstand (6) isintegral with a flange (60) which engages in a slot (80) in the housing(8) to prevent the tibia plate (3) from lifting off the metal base (2)as the said plate slides in rotation about the center of rotation (C,C′).
 18. Knee prosthesis according to claim 1 , characterized in thatthe metal base (2) comprises two upstands (5′ and 9) in the shape of anarc of a circle in opposite directions and centered about the samecenter of rotation (C, C′), while the tibia plate (3) comprises anelement (10) and a housing (12) which are intended to receive the saidupstands (5′ and 9) respectively to allow the said plate to slide inrotation about the center of rotation (C, C′).
 19. Knee prosthesisaccording to claim 18 , characterized in that the upstand (5′) is setout on the external periphery of the horizontal disk (20) of the metalbase (2) so as to engage with a peripheral recess (10) in the tibiaplate (3).
 20. Knee prosthesis according to claim 18 , characterized inthat the upstand (9) is offset from the center of rotation (C, C′) andcomprises a flange (90) which snap-fastens into the housing (12) in thetibia plate (3) to, on the one hand, guide the plate (3) as it slides inrotation about its center (C, C′) and, on the other hand, retain thesaid plate so that it does not lift off the metal base (2).
 21. Kneeprosthesis according to claim 1 , characterized in that the metal base(2) comprises a peripheral upstand (13) in the shape of an arc of acircle integral with a flange (14) directed toward the tibia bonevertical axis (YY′) and a housing (15) set out in the region of thecenter of rotation (C, C′), while the tibia plate (3) has, on itsexternal periphery, a recess (10′) in which there is formed a horizontalslot (16) intended to receive the flange (14) of the said upstand (13)and, on its lower face (30), a stub (17) which engages with the housing(15).
 22. Knee prosthesis according to claim 1 , characterized in thatthe metal base (2) comprises three peripheral upstands (50, 51, 52)extending vertically above the horizontal disk (20), while the tibiaplate (3) has, on its external periphery, three recesses (53, 54, 55)intended to receive the said upstands (50, 51, 52) respectively to allowthe said plate to be guided as it slides in rotation about the center ofrotation (C, C′).
 23. Knee prosthesis according to claim 6 ,characterized in that the pegs (18) are set out in an arc of a circleabout a center of rotation (C, C′), while the tibia plate (3) has ahousing (34) intended to receive the said pegs.
 24. Knee prosthesisaccording to claim 6 , characterized in that the pegs (18) have a centerof rotation (C) which is borne by the tibia bone vertical axis (YY′),while the said peg is a certain distance away from its center ofrotation.
 25. Knee prosthesis according to claim 6 , characterized inthat the pegs (18) have a center of rotation (C′) which is offset fromthe tibia bone vertical axis (YY′), while the said peg is a certaindistance away from its center of rotation.
 26. Knee prosthesis accordingto claim 1 , characterized in that the metal base (2) comprises at leastone upstand or peg (22, 22′, 22″, 26, 29, 5, 6, 5′, 9, 50, 51, 52, 13,15, 18) which engages with a housing (34, 34′, 34″, 35, 7, 8, 10, 12,53, 54, 55, 10′, 17) of the tibia plate (3) so that the said plate canslide in rotation over the metal base (2) only within the limit set bythe difference in size between the said upstand or peg and the saidcorresponding housing.
 27. Knee prosthesis according to claim 26 ,characterized in that the rotational travel between the tibia plate (3)and the metal base (2) is reduced to zero when the dimensions of thehousing (34, 34′, 34″, 7, 10, 10′, 53, 54, 55) are made so as to engagewithout clearance with the upstand (22, 22′, 22″, 5, 5′, 13, 50, 51, 52,18).
 28. Knee prosthesis according to claim 1 , characterized in thatthe short height of the guide means and of their anterior positioning onthe metal base (2) allows the tibia plate (3) to be mounted on the saidbase via a strictly anterior approach, the said plate requiring upwardclearances only by the height of the said guide means.